A system for controlling a braking system is known, for example, from SAE Paper 960991, "Electrohydraulic Brake System--The First Approach to Brake-by-Wire Technology", which describes an electrohydraulic braking system in which an electronic control unit is added onto a hydraulic unit that comprises the hydraulic control components of the braking system. This electronic unit performs control actions on the pressures in the individual wheel brakes as a function of setpoints, and reads measured variables which are related to the wheel brakes, such as wheel brake pressure, wheel speeds, etc. This added-on control unit is connected via a communication system to a second control unit in which the vehicle-specific functions are calculated. These are understood to be, primarily, determination of the setpoints for control actions on the individual wheel brakes, e.g. pressure setpoints, reference braking forces, or torque setpoints. These setpoints are calculated from the driver's braking input, taking into consideration a predefined braking force distribution between the front and rear axles of the vehicle, and optionally taking into consideration wheel- and vehicle-related functions such as braking force distribution as a function of transverse force, antilock braking control, automatic slip control, vehicle dynamics control, etc. If this second control unit, or the communication system between the two control units by means of which setpoints and measured values are exchanged, should fail, satisfactory functioning of the braking system can no longer be maintained in all operating situations.
International Published Patent Application No. WO 95/13946 describes a control system for a braking system which has a central module as well as brake modules associated with braking circuits or wheel groups. The brake modules sense the wheel-specific variables such as wheel speed and braking pressure. One of the brake modules, preferably the one for the front axle, furthermore senses an indicator for the driver's pedal actuation. The sensed values are conveyed via a communication system to the central module, which determines, at least from the operating variables that are delivered, setpoints for the individual wheel brakes. These setpoints are delivered by the communication system to the brake modules for implementation. If the central module fails, the brake module which reads in the actuation signal takes over the determination of setpoints for the wheel brakes in the context of an emergency mode, and adjusts the brakes associated with it accordingly and transmits the setpoints to further brake modules via the communication system. No description is given of a concrete embodiment of the emergency mode, or of a control system for all wheel brakes in the event of failure of the communication system.
German Published Patent Application No. 41 12 388 (corresponding to U.S. Pat. No. 5,281,012) describes a controller for braking force distribution between the front and rear axles. In this system, the braking force distribution is implemented, on the basis of the wheel speed signals, in such a way that the speed difference between the front and rear wheels does not exceed a predetermined threshold value.